The present invention relates to glass sheets suitable for the production of glazing panels capable of being fitted onto motor vehicles and industrial vehicles, and more specifically serving as laminated glazing panels, for example windscreens.
The laminated glazing panels intended by the invention consist, for example, of two glass sheets and at least one transparent sheet of "ionomer resin".
The expression "ionomer resin" denotes a resin which can be extruded and contains ethylene/carboxylic acid or alpha-olefin/carboxylic acid copolymers, these copolymers being crosslinked by ionic reaction. This type of resin is in particular described in Patents EP-483,087, EP-191,088, U.S. Pat. Nos. 4,619,973, 4,732,944 and 4,906,703 to which reference will be made for the choice of ionomer resins suited to the production of the laminated glasses according to the invention. As ionomer resin, it is possible to use, in particular, resins based on copolymers which are crosslinked by ionic reaction and are obtained by a combination of ethylene, styrene or propylene monomers and of acrylic or methacrylic acid or maleic anhydride monomers. The ionomer resin(s) used in the laminated glazing panel according to the invention is (are) generally based on ethylene (or alpha-olefin)/acrylic (or methacrylic) acid copolymer(s) crosslinked by ionic reaction. The ionomer resins available commercially generally contain the acid copolymers mentioned above, possible partly neutralized, or metal or amine salts of the said acid copolymers (in particular, sodium or zinc ionomers are found).
The sheets of ionomer resin may in particular be obtained by casting, extrusion, etc. Several structures of glazing panels according to the invention may be envisaged, such as the structures described in Patents EP-0,191,088 or EP-0,483,087.
The glazing panels used for this type of application must meet the legal requirements pertaining to their light transmission. Thus, these glazing panels must exhibit an overall light transmittance under illuminant A(TL.sub.A) at least equal to 70%.
Since the glazed are of motor vehicles is currently very large and the customer requirements in terms of comfort are ever increasing, the constructors of these vehicles seek all means which make it possible to lessen the sensation of heat experienced by the passengers exposed to solar radiation. However, at the same time, the motor-vehicle constructors are seeking to lighten as far as possible all the glass fittings.
In order to maintain a high light transmission in the visible part of the spectrum while still absorbing as much as possible of the rest of the solar energy, it is known to introduce iron into the composition of the glass used for manufacturing the sheets. Iron is present in the glass both in the form of ferric oxide (Fe.sub.2 O.sub.3) and ferrous oxide (FeO). The presence of Fe.sub.2 O.sub.3 makes it possible to absorb the UV radiation and that having short wavelengths in the visible part of the spectrum; on the other hand, the presence of FeO makes it possible to absorb radiation in the near IR and that corresponding to long wavelengths in the visible range. Although increasing the iron content, in its two oxidized forms, increases the absorption of radiation at both ends of the visible spectrum, this effect is achieved to the detriment of the light transmission.
To date, various solutions have been proposed in order to make best use of the radiation-absorption capability of iron oxides whilst nevertheless maintaining the highest possible light transmission.
Thus, Patent EP-B-297,404 describes and claims silica-soda-lime glasses whose total iron content, expressed in the form of Fe.sub.2 O.sub.3 is between 0.45 and 0.65%. These glasses are founded under conditions such that at least 35% and preferably at least 50% of the total iron is in the form of FeO. The increase in the FeO content thus obtained makes it possible to increase the absorption of the glasses in the infrared and to decrease the overall energy transmittance (T.sub.E). However, when a glass is founded in the presence of sulphur under highly reducing conditions, the glass takes on an amber colour due to the formation of chromophores resulting from the reaction between the sulphur and the ferric iron. In order to avoid this, it is therefore necessary to remove the sulphates from the charge and, since the sulphur content in a glass is never zero, to ensure that the percentage of ferric iron remains low, which results in the total iron content being strictly limited. It follows that the ability of these glasses to absorb UV radiation is poor.
It is also known to manufacture glasses which, by virtue of a higher total iron content than that recommended by the European patent mentioned above, reconciles good light transmission with good absorption of both infrared and ultraviolet radiation.
Thus, U.S. Pat. No. 5,214,008 describes glasses divested of ceric oxide and other oxides of this type, these glasses containing between 0.7 and 0.95% by weight of total iron expressed in the form of Fe.sub.2 O.sub.3. These glasses are founded in conventional furnaces, using standard glass-forming batch materials. The degree of oxidation-reduction of the glass is controlled through the introduction of carbon and of sodium sulphate into the charge.
This degree of oxidation-reduction varies within precise limits such that the iron in the form of FeO in the glass varies from 0.19 to 0.24% by weight, the said glass having, for a thickness of between 3.7 and 4.8 millimeters, a light transmittance of greater than 70%, an ultraviolet transmittance of less than 38% and an overall energy transmittance of less than 44.5%.
Other silica-soda-lime glass compositions make it possible to obtain, for a given thickness, a light transmittance at least equal to 70% and good absorption of infrared and ultraviolet radiation. This is particularly the case for those glasses described in Patent Applications EP-A-488,110 and WO-91/07356. Apart from the iron oxides, the glasses recommended by these patent applications contain ceric oxide and titanium oxide.